1. Field of the Invention
The invention relates to a radar provided with a clutter elimination device or system. The "clutter" referred to herein is noise caused on a radar display by the presence of a profusion of echoes from fixed targets such as sea waves, atmospheric conditions, some ground configurations, bird or insect flights, or chaff.
2. Description of the Prior Art
To better understand the invention, consider first the general configuration of a radar system as shown in FIG. 1A. Schematically, a radar system comprises an oscillator which provides a signal at a frequency f in the range of some GHz. This signal is amplified by an amplifier 2 which emits signal bursts (such as the bursts 9a and 9b in FIG. 1B), each of which comprises several cycles of energy at the high frequency f, for a short pulse duration T of, for example, about 500 nanoseconds. This signal is transmitted toward a target by an antenna 3; that same antenna 3 or another neighboring antenna 4 receives a returning echo from the target. This echo is transmitted from the receiving antenna to the first input of a mixer 5. The second input of the mixer 5 receives the output of a stable or stabilized oscillator 6 providing a carrier at a frequency f+f.sub.i. The frequency f.sub.i is called the "intermediate frequency" and is typically in the range of some tens or hundreds of MHz. In the following examples, the intermediate frequency f.sub.i is 30 MHz, but of course this does not constitute a limitation of the invention. At the output of the mixer 5 there appears a signal resulting from the interaction between the echo and the local oscillator signal at frequency f+f.sub.i. If there is no Doppler effect shifting the frequency of the echo, this output signal is at a frequency equal to f.sub.i. The signal at the output of the mixer 5 is transmitted to a filter 7 and thence to a decoder and display system 8. FIG. 1C shows very schematically the signal 11 at the output of the mixer 5; this signal has substantially the same envelope as that of signal 9 emitted by the radar transmitter--a train of pulses of duration T--but the frequency of the carrier within the pulse envelope is much lower than that of the oscillator 1, being close to the frequency f.sub.i.
The main purpose of the filter 7 is to transmit the returning pulse while eliminating as far as possible the white noise outside the radar pulse spectrum. It is known that, in order to obtain such a result, the filter must have a transfer function of the type (sin x)/x with x=(w-w.sub.i).pi.T. This (sin x)/x comprises, as is well known, a principal lobe surrounded with a great number of lateral lobes having a lesser amplitude. Therefore, the filter 7 as used in practice generally has a response curve or transfer function the shape of which is as close as possible to the shape of the principal lobe of a (sin x)/x function. A typical filter response curve is shown at 12 in FIG. 2. As a numerical example, if the intermediate frequency is equal to 30 MHz and the pulse-time duration is equal to 500 nanoseconds, this filter will have transmission maxima at 30 MHz and transmission minima at 28 and 32 MHz. Generally, it has been accepted that such a filter characteristic, which permits a good elimination of white noise, also permits a satisfactory suppression of clutter noise. When clutter noise is too high, the usual attempt to eliminate or suppress it is made not at the level of this filter (i.e., not on a pulse-by-pulse basis), but, rather, at the level of the decoder and display system; to that end, pulses are stored as they are received and comparisons are made between the information content of successive radar echoes. These classical clutter elimination devices, however, usually are complex and not very efficient.